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KVM: Get writable mapping for __kvm_vcpu_map() only when necessary

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When creating a memory map for read, don't request a writable pfn from the
primary MMU.  While creating read-only mappings can be theoretically slower,
as they don't play nice with fast GUP due to the need to break CoW before
mapping the underlying PFN, practically speaking, creating a mapping isn't
a super hot path, and getting a writable mapping for reading is weird and
confusing.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-35-seanjc@google.com>
This commit is contained in:
Sean Christopherson 2024-10-10 11:23:36 -07:00 committed by Paolo Bonzini
parent 365e319208
commit 2e5fdf60a9
1 changed files with 1 additions and 1 deletions
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2
virt/kvm/kvm_main.c
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@ -3116,7 +3116,7 @@ int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map,
struct kvm_follow_pfn kfp = {
.slot = gfn_to_memslot(vcpu->kvm, gfn),
.gfn = gfn,
.flags = FOLL_WRITE,
.flags = writable ? FOLL_WRITE : 0,
.refcounted_page = &map->pinned_page,
.pin = true,
};